Resist, including photoresist, is a radiation sensitive (e.g., light radiation sensitive) material used to form a patterned layer on a substrate (e.g., a semiconductor wafer) during semiconductor device fabrication. After exposing a portion of a resist coated substrate to radiation, either the exposed portion of the resist (for positive resist), or the unexposed portion of the resist (for negative resist) is removed to reveal the underlying surface of the substrate, leaving the rest of the surface of the substrate coated and protected by resist. Resist may be more generally referred to as a masking material. Other fabrication processes such as ion-implanting, etching, or depositing may be performed on the uncovered surface of the substrate and the remaining resist. After performing the other fabrication processes, the remaining resist is removed in a strip operation.
In ion-implantation, dopant ions (e.g., ions of boron, boron difluoride, arsenic, indium, gallium, thallium, phosphorous, germanium, antimony, xenon or bismuth) are accelerated toward a substrate to be implanted. The ions are implanted in the exposed regions of the substrate as well as in the remaining resist. Ion-implantation may be used, for example, to form implanted regions in the substrate such as the channel region and source and drain regions of transistors. Ion-implantation may also be used to form lightly doped drain and double diffused drain regions. However, ions implanted in the resist may deplete hydrogen from the surface of the resist causing the resist to form an outer layer or crust, which may be a carbonized layer that is harder than the underlying portion of the resist layer (i.e., the bulk portion of the resist layer). The outer layer and the bulk portion have different thermal expansion rates and react to stripping processes at different rates. High dose ion-implanted resist may cause severe hardening or crusting of the resist resulting in relatively large differences between the outer layer and bulk portion in, for example, differences in thermal expansion rates, solubilities and other chemical and physical characteristics.
One type of transistor is known as a field-effect-transistor (FET). An FET may also be known as a metal-oxide-semiconductor FET (MOSFET), although MOSFET is a misnomer for FETs having a silicon gate instead of a metal gate. FET transistors comprise a source region, a drain region, a channel region between the source and drain regions, a gate insulator above the channel region and a gate electrode above the gate insulator. In early FETs from very early technologies, gate electrodes typically comprised metal. In later technologies, gate electrodes typically comprised semiconductor silicon (e.g., in the form of polysilicon). Silicon was used because silicon is compatible with silicon dioxide used as the gate insulator, and because silicon could tolerate high temperatures that were useful for fabricating FETs and integrated circuits that included FETs. However, some very recent technologies are again using metal gate electrodes. Metal has the advantage of having less electrical resistance than polysilicon, thus reducing signal propagation times. Furthermore, in very recent technologies having transistor dimensions that are smaller that dimensions of preceding technologies, it is necessary to make the gate dielectric layer very thin (e.g., one nanometer). Very thin gate dielectric layers may cause a problem in polysilicon gate electrodes, called poly depletion, where a depletion layer is formed in the gate polysilicon electrode next to the gate dielectric when the channel region of the transistor is in inversion. To avoid poly depletion, a metal gate is desired. A variety of metal gates materials may be used, usually in conjunction with relatively high dielectric constant gate insulator materials, known as high-k dielectrics. Examples of metal gate materials include tantalum, tungsten, tantalum nitride, and titanium nitride (TiN).